This 2” Celestron 31mm Luminos is an excellent low power eyepiece with an exceptionally wide field of view, one so wide you have to move your head from side to side to see it all. The six-lens design is primarily designed for no-holds-barred low power deep space use. It is particularly well-suited to get the maximum image brightness and field of view out of big Dobsonian scopes, but will work well with any telescope that can use 2” eyepieces, even an f/15 Maksutov.

This eyepiece has a very large 6.9mm exit pupil with an f/4.5 scope, larger than some observers’ eyes can dilate. That can effectively waste some of the scope’s light gathering capacity, as some of the full light cone will fall on the older observer’s iris, rather than entering his or her eye. Even so, some observers willingly trade a little light gathering for the immense field of view of this 82° apparent field eyepiece. This eyepiece very much provides a true “picture window of space” view.

You can use the calculator below to see just how wide the field will be with your particular telescope. For example, it will give you a 2.22° field at 37x with a 10" f/4.5 Dobsonian, more than enough to take in all of the Pleiades at one time. This gives you over eight minutes of uninterrupted observing with such a scope while an object drifts from one side of the eyepiece’s flat field to the other. That’s plenty of time for multiple observers to look at the same object, or for you to absorb as much detail as possible, before having to reposition the scope.

With a factory-specified eye relief of 27mm, the 31mm Luminos will provide fully unvignetted views for most eyeglass wearers. However, since the 82° field is so wide that you can’t see the full field without having to move your head from side to side to take it all in anyway, vignetting in the conventional sense is not a problem in any case.

At 40 ounces, the 2" 31mm Luminos is a very substantial eyepiece. Be sure that your star diagonal is very firmly locked in place on your scope before inserting this eyepiece to avoid the chance of the diagonal swiveling unexpectedly and the eyepiece falling to the ground.

The retractable eyeguard raises by turning the broad rubber grip ring around the eyepiece barrel clockwise and lowers by turning the ring back counterclockwise. The eyeguard mechanism may be a little stiff in operation the first time you raise it.

Eye relief is the distance from the last surface of the eye lens of an
eyepiece to the plane behind the eyepiece where all the light rays of
the exit pupil come to a focus and the circular image is formed,
sometimes called the “Ramsden Disk.” This is where your eye should be
positioned to see the full field of view of the eyepiece. If you must
wear glasses because of astigmatism, you’ll usually need at least 15mm
of eye relief or longer if you want to see the full field of view with
your glasses on.

A note on our eye relief figures: Quite
often, our eye relief figures will differ from those of the
manufacturer. This is because we measure the “usable” eye relief, while
the manufacturers specify their usually-longer (but technically correct)
“designed” eye relief.

The eye lens of the eyepiece is normally
recessed below the rubber eyeguard or rubber rim of the eyepiece to keep
the lens from being scratched during use. An eyepiece might have a
“designed” eye relief of 15mm (and the eye relief will truly measure
15mm from the eye lens to where the image forms). However, if the eye
lens is recessed 3mm below the eye guard, the Ramsden Disk forms only
12mm above the eyepiece body (the 15mm “designed” eye relief, less the
3mm of eye relief made unusable by having the eye lens recessed into the
body of the eyepiece). This “usable” eye relief of 12mm (measured from
the rolled-down eyeguard – the closest point you can get your eye to the
eye lens – to where the image forms) is the eye relief figure we would
measure and list in this website.

Why is it important to list the “usable” eye relief? For
those people who don’t wear eyeglasses while observing, a few mm
difference between the eye relief they expect from the manufacturer’s
literature and the shorter eye relief they actually get in real life
doesn’t mean a lot. They can simply move a little closer to the eyepiece
to see the full field, and never realize that the eye relief is a
little shorter than they expected. However, some people must wear
eyeglasses while observing, because of severe astigmatism. These
observers can’t move closer to the eyepiece if the eye relief is shorter
than expected because their glasses get in the way. For these people,
the real life “usable” eye relief is more important than the technically
correct but sometimes not fully usable “designed” eye relief. We
measure and list the actual usable eye relief so that people in the real
world can pick the eyepieces that will work best for them.

This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

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Clear skies,
Astronomics

This 31mm 2" Celestron Luminos eyepiece has an immense 82° field of view for low power "picture window" views with 2" focuser Dobsonians and short focal length refractors . . .